1. Field of the Invention
This invention relates to polyurethane elastomers and methods for their preparation and is more particularly concerned with solution grade polyurethane elastomers.
2. Description of the Prior Art
Polyurethane elastomers derived from 4,4'-methylenebis (phenylisocyanate) (MDI) and polyester diols are well known in the art, see for example U.S. Pat. No. 2,625,535 and U.S. Pat. No. 3,298,995. Solution grade polyurethane elastomers are also well known in the art; see U.S. 2,871,218, 3,298,995 and 3,632,845. In general it has been recognized previously that, in order to obtain a soluble thermoplastic polyester polyurethane elastomer possessing good physical properties, such as good tensile strength and elongation at a given hardness, the aromatic diisocyanate be reacted with a combination of a linear hydroxyl terminated polyester and a saturated short chain aliphatic diol or other form of difunctional extender. The prior art has taught that the ratio of isocyanate groups to reactive hydroxyl groups be essentially unity; see for example U.S. Pat. No. 2,871,218.
I have now found, quite surprisingly, that such solvent soluble thermoplastic polyurethanes can be formulated without the usual short chain diol, by relying on the structure of the macroglycol - MDI block to obtain the desired combination of solvent solubility and good properties in a given hardness range. I have also found quite unexpectedly, that the ratio of NCO:OH must be less than unity. The molecular weight ranges of the polyester diols employed in the present invention, in general have not hitherto been considered high enough to be useful for the preparation of solvent soluble thermoplastic polyurethane elastomers.